1. Field of the Invention
The present invention relates to a color image forming apparatus in which a plurality of image forming units are arranged around a photosensitive member and in which a plurality of colors of toner images are superimposed on the photosensitive member to obtain a color image.
2. Description of the Related Art
In color image forming apparatuses, various systems have been adopted in order to achieve miniaturization, speeding-up of processes, and enhancement of position accuracy in superimposing images of color components. For example, there is an image forming apparatus of electronic photography, in which toner images developed by toners formed of four coloring materials of yellow (Y), magenta (M), cyan (C), and black (BK) are superimposed on one photosensitive member to obtain a full-color image.
As one example of a full-color image forming apparatus, there is a process (Image On Image process, hereinafter referred to as IOI process) in which charging, exposing, and developing are successively repeated on one photosensitive member for each color of toner, and monochromatic toner images are superimposed on the photosensitive member, and thereafter collectively transferred onto a transfer member. A color image forming apparatus which performs the IOI process is utilized in a color printer, or a color copying machine, and in on-demand printing or color proof in a printing field.
For example, in Jpn. Pat. No. 3208670 (pages 8, 9, FIGS. 1 and 9), an apparatus has been described in which a charging potential of the photosensitive member, or an image density is detected to control a charging device, an exposing device, or a developing device. When the surface potential on a photosensitive drum is detected by a single charging electrometer, the charging device or the exposing device is controlled in such a manner that the surface potentials of unexposed and exposed portions indicate present reference values. When the density of the toner image on the photosensitive drum is detected, a developing bias is controlled in such a manner as to set the toner image density to the reference value.
For example, in Jpn. Pat. No. 2769704 (pages 3, 4, FIGS. 1 and 2), an apparatus is described in which a plurality of surface potential sensors are used in order to calculate the surface potential in a developing device position of the photosensitive member. Potentials in a plurality of developing unit positions are calculated from potential differences on the photosensitive member, detected by first and second surface potential sensors, a charging amount of the charging unit is controlled in such a manner as to adjust the surface potentials in a plurality of developing unit positions into set values, and a color image is obtained.
However, it is difficult to apply the technique described in the Jpn. Pat. Nos. 3208670 or 2769704 to the IOI process, because it is necessary to control image formation in consideration of fluctuations or differences of characteristics in a plurality of elements such as charging and exposing devices.
Especially in the IOI process, a charging step of the next stage has to be performed before influences of the charging by the charging device of the previous stages are reduced. Additionally, the next image forming process is performed in such a manner as to superimpose an image upon a toner image formed in the previous stage. Therefore, if the influences on charging characteristics by the image forming process in the previous stage are not considered, correct image forming control cannot be performed, and there is a problem that an image quality drops.
In the color image forming apparatus which performs the IOI process, a potential on the surface of the photosensitive member changes even in the use on the same conditions by changes of environments such as ambient temperature, humidity, and temperature in the apparatus, a drop of capability of a charging device and a change of a characteristic such as a resistance value of the surface of the photosensitive member after the use for a long time. Further, because of changes of characteristics of a developer with time, it is difficult to maintain image qualities in broad senses, such as density and color of a toner image, constantly in certain states.